1. Field of the Invention
The present invention relates to magnetic flow meters and a method of making magnetic flow meters.
2. Description of the Related Art
A conventional utility water meter can include a solid-state flow transducer. Such a flow transducer is a magnetic flow transducer of a kind that is well known and shown by way of example in the cross-sectional view of FIG. 1.
In FIG. 1, a flow tube 101 incorporates a magnetic transducer 109 comprising a pair of electrodes 102 disposed across a diameter of the pipe 101, with at least part of one surface of each electrode 102 in intimate contact with the fluid 108 in the pipe 101. Magnetic pole pieces 103 are disposed across the orthogonal diameter of the pipe 101 and linked by a magnetic circuit 104. As is well known in the art, the magnetic field 107 imparts a force on charged species moving with a bulk medium (ions in the case of water), causing the charged species to migrate in a direction orthogonal to both the magnetic field and the direction of bulk fluid motion. The mutual displacement of oppositely charged species results in an electric field along the direction of migration which builds up until the electrostatic force on a given ion is balanced by the magnetic force. Since the magnetic force depends implicitly on the bulk medium flow velocity, measurement of the opposing electric field (or potential difference) provides a convenient means for determining the flow rate, while integration over time allows the total volume that has passed through the tube to be calculated. Circuitry for processing the electrode signals to obtain such measurements is well known in the art and consequently not described in greater detail here.
As is also well known, it can be advantageous to alternate the applied magnetic field, so as to overcome various limitations of a static field measurement. One such limitation is imposed by the nature of the electrodes used to measure the electrical potential difference in the fluid. An ideal electrode will form a perfect electrical connection to the fluid, with no energy barrier to the exchange of charge either way across the solid-liquid interface.
To understand the frequency-dependent behavior of the electrodes, it is useful to consider a simple electrical model of FIG. 2 that is often applied to the solid-liquid interface 201, consisting of a resistor 202 in parallel with a capacitor 203. The direct exchange of charged species between the solid 204 and the liquid 205 is signified by the flow of current through the resistor 202, while the capacitor 203 represents the tendency of charged species to accumulate in the vicinity of the interface, without actually crossing it. At frequencies substantially above 1 Hz, the capacitor 203 generally provides the easier route for the flow of a small-signal current through a solid-liquid interface.
In the device of FIG. 1, an alternating magnetic field is achieved by means of coils 105 wound around part of the magnetic circuit 104 and supplied with a suitable alternating current waveform. Furthermore, to reduce power consumption, it is known to provide the magnetic circuit 104 with one or more elements 106 exhibiting magnetic remanence so that the coils 105 need only be energized when it is required to change the state of the magnetic field.
An exemplary magnetic flow transducer, designed to further reduce power consumption, is described in U.S. Pat. No. 7,472,605, incorporated herein by reference in its entirety. FIG. 3 shows an electrode of a first embodiment of such an exemplary magnetic flow transducer comprising a metal element 301 (e.g. a wire, a plate, or a film completely covering an underlying conductor) coated with an ionic compound 302 of the same metal, which is sparingly soluble in the fluid of the flow to be measured 303.
The metal 301 can be silver, with the accompanying compound 302 being silver chloride. A fully-reversible, galvanic exchange of charge occurs between the fluid 303 and the metal 301 by means of silver ions crossing the phase boundary between the solid silver electrode 301 and the hydrated silver chloride layer 302. The electrical potential across the interface is defined by the Nernst equation, which in turn depends on the surface concentration of AgCl and the liquid concentration of Cl ions. While these quantities will not be constant, they may normally be expected to vary on a timescale much greater than the period of the alternating magnetic field. In accordance with the invention, the timescale of this variation is significantly longer than for an electrode surface which does not possess a controlled ionic exchange mechanism. Accordingly, operation at a lower frequency is facilitated because the noise energy is reduced.
FIG. 4 illustrates an apparatus for implementing a method for boosting the quantity of ionic compound for situations where spontaneous reactions are insufficient. A third electrode 404 is provided for making contact to the fluid 403, in addition to the measuring electrodes 401 and 402. The third electrode 404 need not be made of the same material as the measuring electrodes 401 and 402 (e.g. graphite or steel would suffice). At intervals determined by control electronics 405, a substantial potential is applied between the third electrode 404, and the measuring electrodes 401 and 402. For the silver chloride system described above, the measuring electrodes 401 and 402 would both be held at a positive potential with respect to the third electrode 404, sufficient to cause a quantity of the silver in the measuring electrodes 401 and 402 to react with negatively charged ionic species in the vicinity. The ionic species would preferably be chloride, and the potential applied between the electrodes may be chosen to favor such a reaction.
FIG. 5 shows an electrode prepared using the above method and comprising a flat section of silver 501 coated electrochemically with a thin film of silver chloride 502. Positioned in direct contact with the flowing liquid 503, and preferably flush with the wall of the flow tube so as to reduce turbulence and the corresponding measurement noise and uncertainty, its surface will tend to be abraded over time by particulates in the liquid, thereby helping to maintain an active electrode surface.